Establishing how the response properties of neurons in visual cortex emerge from the synaptic interactions of distinct circuit elements remains one of the fundamental challenges for visual neuroscience and is central to understanding the neural basis of visual perception and disorders of visual function. The experiments in this proposal continue our analysis of the functional organization of circuits in layer 2/3 of visual cortex by employing state of the art techniques to determine how the well-developed network of horizontal connections within layer 2/3 interacts with feedforward inputs from cortical layer 4 to shape the visual response properties of layer 2/3 neurons. The first aim employs in vivo 2-photon imaging of calcium signals in combination with retrograde labeling techniques to determine, with single cell resolution, the visual response properties of the neurons that contribute to the horizontal network in layer 2/3. The second aim employs in vivo optogenetic control of the activity of layer 2/3 neurons to assess how modulating the activity of horizontal inputs impacts the visually driven responses of layer 2/3 neurons. In the final aim, optogenetic techniques will be combined with in vivo intracellular recordings to assess the synaptic interactions that account for the contribution of horizontal connections to the response properties of layer 2/3 neurons. Taken together, these experiments should provide a wealth of new insights into the functional organization of the neural circuits in visual cortex-information that is essential for understanding the neural basis of visual perception and provides the foundation for evaluating disruptions of normal cortical function that are common in neurological and neuropsychiatric disorders.